Background

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare, chronic, debilitating disorder that most frequently presents in early adulthood and usually continues throughout the life of the patient. PNH results in the death of approximately 50% of affected individuals due to thrombotic complications and, until recently, had no specific therapy.

The name of the disorder is a descriptive term for the clinical consequence of red blood cell (RBC) breakdown with release of hemoglobin into the urine, which manifests most prominently as dark-colored urine in the morning (see image below).

This series of containers holds urine of a patient with paroxysmal nocturnal hemoglobinuria, showing the episodic nature of the dark urine (hemoglobinuria) during intravascular hemolysis, usually occurring at night. Early morning urine is cola-colored. This may occur at different times of the day and vary from patient to patient. Permission to use this image has been granted by the American Society of Hematology Slide Bank, 3rd edition.

The term "nocturnal" refers to the belief that hemolysis is triggered by acidosis during sleep and activates complement to hemolyze an unprotected and abnormal RBC membrane. However, this supposition was later disproved. Hemolysis has been shown to occur throughout the day and is not actually paroxysmal, but the concentration of urine overnight produces the dramatic change in color.

PNH has been referred to as "the great impersonator" because of the variety of symptoms observed during its initial manifestation and course. This variety reflects the contributions of the following three underlying pathophysiologic events
[1, 2, 3, 4] :

An acquired intracorpuscular hemolytic anemia due to the abnormal susceptibility of the RBC membrane to the hemolytic activity of complement

Thromboses in large vessels, such as hepatic, abdominal, cerebral, and subdermal veins

A deficiency in hematopoiesis that may be mild or severe, such as pancytopenia in an aplastic anemia state

Pathophysiology

Paroxysmal nocturnal hemoglobinuria (PNH) was previously classified as purely an acquired hemolytic anemia due to a hematopoietic stem cell mutation defect. This classification was abandoned because of the observation that surface proteins were missing not only in the RBC membrane but also in all blood cells, including the platelet and white cells.

The common denominator in the disease, a biochemical defect, appears to be a genetic mutation leading to the inability to synthesize the glycosyl-phosphatidylinositol (GPI) anchor that binds these proteins to cell membranes.
[4, 5, 6] The corresponding gene PIGA (phosphatidylinositol glycan class A) in the X chromosome can have several mutations, from deletions to point mutations.
[7]

Due to its location on the X chromosome, and X inactivation in female somatic cells, only one mutation is required in either males or females to abolish the expression of GPI-linked proteins. Most type II PNH cells (total lack of GPI-linked protein) are due to a frame shift mutation occurring in the early hematopoietic progenitor cells, resulting in the same mutation in all blood cell lines.

The essential group of membrane proteins that are lacking in all hematopoietic cells are called complement-regulating surface proteins, including the decay-accelerating factor (DAF), or CD55
[8] ; homologous restriction factor (HRF), or C8 binding protein; and membrane inhibitor of reactive lysis (MIRL), or CD59.
[9] All of these proteins interact with complement proteins, particularly C3b and C4b, dissociate the convertase complexes of the classic and alternative pathways, and halt the amplification of the activation process.

The absence of these regulating proteins results in uncontrolled amplification of the complement system. This leads to intravascular destruction of the RBC membrane, to varying degrees

Breakdown of RBC membranes by complement leads to the release of hemoglobin into the circulation. Hemoglobin is bound to haptoglobin for efficient clearance from the circulation. After saturating the haptoglobin, free hemoglobin circulates and binds irreversibly with nitric oxide (NO), depleting NO levels in peripheral blood.

Because NO regulates smooth muscle tone, depletion of NO levels leads to smooth muscle contraction with consequent vasoconstriction, constriction of the gut, and pulmonary hypertension. Resultant symptoms may include the following:

Abdominal pain

Bloating

Back pain

Headaches

Esophageal spasms

Erectile dysfunction

Fatigue

Progressive chronic renal failure can occur after several years of hemoglobinuria from the acute tubulonecrosis effects of heme and iron (pigment nephropathy), decreased renal perfusion from renal vein thrombosis, and tubular obstruction with pigment casts. Patients with PNH experience a high incidence (40%) of thrombotic events (mostly venous) in large vessels (cerebral, hepatic, portal, mesenteric, splenic, and renal veins) and, most recently recognized, arterial thrombosis.

The pathophysiology of thrombophilia in PNH is not fully understood, but the increased incidence during hemolytic episodes suggest a direct relationship with the hemolytic process. Increased procoagulant and fibrinolytic activity, suggesting increased fibrin generation and turnover, increased plasma levels of urokinase-type plasminogen activator and platelets deficient in GPI-linked proteins activated by complement, have been implicated. However, none of these identified platelet and coagulation abnormalities can fully explain the hypercoagulable state in PNH.

Bone marrow failure is present in all patients with PNH, even when peripheral blood counts are normal and the bone marrow is hypercellular. The degree of marrow failure may vary from severe aplastic anemia to a decrease in the number of hematopoietic stem cells. Bone marrow failure is possibly due to similar destruction by complement, but the cause or causes are still poorly understood.

Venous thrombosis usually manifests as a sudden catastrophic complication, with severe abdominal pain, a rapidly enlarging liver, and ascites (Budd-Chiari syndrome). This thrombosis may be due to a lack of CD59 on platelet membranes, which induces platelet aggregation and is highly thrombogenic, particularly in the venous system.

Deficient hematopoiesis may occur due to diminished blood cell production by a hypoplastic bone marrow; thus, patients have a 10-20% chance of developing aplastic anemia in their course. In turn, PNH eventually develops in 5% of patients with aplastic anemia.
[7, 10] The nature of the pathogenetic link between those two diseases is still unknown.

Pathogenesis

When intravascular hemolysis occurs, RBCs release hemoglobin (Hb) into the plasma. This free Hb is rapidly dimerized and bound by serum protein haptoglobin and rapidly removed by macrophages, which then degrades it after endocytosis. Since haptoglobin is not recycled, large amounts of free Hb can deplete the body's supply, leaving the excess Hb free in the plasma.

When the capacity to manage and degrade free Hb during acute or chronic hemolysis is reached, levels of Hb and heme increase in the plasma and urine. Plasma Hb has the ability to scavenge nitric oxide (NO), resulting in rapid consumption of NO and clinical sequelae of NO depletion. NO plays a major role in vascular homeostasis and has been shown to be a critical regulator of basal and stress-mediated smooth muscle relaxation and vasomotor tone, endothelial adhesion, and platelet activation and aggregation.

Thus, the clinical consequences of excessive cell-free plasma Hb levels during intravascular hemolysis or the administration of Hb preparations include dystonias involving the gastrointestinal, cardiovascular, pulmonary, and urogenital systems, as well as clotting disorders. Many of the clinical sequelae of intravascular hemolysis in a prototypic hemolytic disease, PNH, are readily explained by Hb-mediated NO scavenging.

Paroxysms or episodes of symptoms occur during sudden and marked increases in the rate of intravascular hemolysis. These episodes can be precipitated by infections, drugs, or trauma or they can occur spontaneously. During paroxysms, PNH patients exhibit symptoms consistent with smooth muscle perturbation through the release of Hb and NO scavenging, including abdominal pain, esophageal spasms, and erectile dysfunction.

NO also plays an important role in the maintenance of normal platelet functions through the down-regulation of platelet aggregation and adhesion and the regulation of molecules in the coagulation cascade. Accordingly, the long-term consumption of NO by plasma Hb has been implicated in the formation of clots in PNH patients.
[11]

Thrombosis is the most common cause of death in persons with PNH, accounting for 50% of the mortality from the disease. The most frequent sites of thrombosis include the hepatic, pulmonary, cerebral, and deep and superficial veins, as well as the inferior vena cava. Approximately 15%-20% of thrombosis in PNH is arterial.
[12] Interestingly, there is a close correlation between thrombosis and a large PNH clone, and clone size correlates with hemolytic rates.

The reason for the propensity for thrombosis is not entirely clear. Intravascular hemolysis may provide altered membrane surfaces upon which coagulation may be initiated.

More likely, it is the effect of the activation of complement on platelets and perhaps endothelial cells. In platelets, the deposition of C9 complexes on the surface stimulates their removal by vesiculation; these vesicles are very thrombogenic. Because PNH platelets lack the mechanism for down-regulating C9 deposition (ie, CD59), even a minimal stimulus from activated complement results in a greatly increased production of these vesicles.
[13]

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Etiology

For some time, paroxysmal nocturnal hemoglobinuria (PNH) has been known to result from somatic mutations in the PIGA gene, which encodes phosphatidylinositol glycan class A (PIGA). These mutations result in hematopoietic stem cells that are deficient in glycosyl-phosphatidylinositol anchor protein (GPI-AP). Nonmalignant clonal expansion of one or several of these stem cells leads to clinical PNH.

Shen et al have identified additional somatic mutations associated with PNH. These mutations were in genes known to be involved in myeloid neoplasm pathogenesis, including TET2, SUZ12, U2AF1, and JAK2. Clonal analysis indicated that these additional mutations arose either as a subclone within the PIGA-mutant population or had occurred prior to PIGA mutation.
[14]

The clinical pathology in PNH may actually be an epiphenomenon resulting from an adaptive response to injury, such as an immune attack on hematopoietic stem cells.

In PNH, the peripheral blood and bone marrow is a mosaic composed of GPI-AP+ and GPI-AP– cells; with GPI-AP–, cells can be derived from multiple mutant stem cells. The GPI-AP– mutant cells may appear to dominate hematopoiesis in PNH by providing a proliferative advantage under some pathologic conditions. For example, if damage to stem cells causing bone marrow failure is mediated through a GPI-linked surface molecule, the PNH cells lacking these molecules will survive. The close association of PNH with aplastic anemia and myelodysplastic syndrome (MDS) suggests that the selection process arises as a consequence of this specific type of bone marrow injury.

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Epidemiology

Paroxysmal nocturnal hemoglobinuria (PNH) is an uncommon disorder of unknown frequency both in the United States and worldwide. There is little information on the incidence of PNH, but the rate is estimated to be 5-10 times less than that of aplastic anemia; thus, PNH is a rare disease.

Attempts to determine the incidence more accurately and to learn more about the natural course of the disease are currently in progress under the auspices of the PNH Registry—"a worldwide collection of data aiming at improving and sharing the understanding of PNH for a better management of patients with PNH".

It has been suggested that, like aplastic anemia, PNH may be more frequent in Southeast Asia and in the Far East.

Mortality/Morbidity

The disease process of PNH is insidious and has a chronic course, with a median survival of about 10.3 years. Morbidity depends on the variable expressions of hemolysis, bone marrow failure, and thrombophilia that define the severity and clinical course of the disease.

A study of the first 1610 patients enrolled in the International PNH Registry found that overall, 16% of patients had a history of thrombotic events and 14% a history of impaired renal function. Frequently reported symptoms included the following
[15] :

Fatigue (80%)

Dyspnea (64%)

Hemoglobinuria (62%)

Abdominal pain (44%)

Chest pain (33%)

Patients also reported impairment in quality of life from their disease, with 17% stating that they were not working or were working less because of PNH.

In several large studies, the main cause of death in patients with PNH was venous thrombosis, followed by complications of bone marrow failure; however, spontaneous long-term remission or leukemic transformation of the PNH clone has been reported and well documented.

The median survival after diagnosis was 10 years in a series of 80 consecutive patients seen at the Hammersmith Hospital in London who were treated with supportive measures, such as oral anticoagulant therapy after an established thrombosis, and transfusions.
[16] Sixty patients died; of 48 patients whose cause of death was known, 28 died from venous thrombosis or hemorrhage. Thirty-one individuals (39%) had one or more episodes of venous thrombosis during their illness.
[16] No leukemic transformations occurred in this series.

Twenty-two of the 80 patients (28%) survived for 25 years.
[16] Of the 35 patients who survived for 10 years or more, 12 had spontaneous clinical recovery; no PNH-affected cells were found among the RBCs or neutrophils during their prolonged remission, but a few PNH-affected lymphocytes were detectable in 3 of 4 patients tested.
[16]

Race

The differences of PNH) among races were shown in a study that compared 176 American patients seen at Duke University and 209 patients from Japan.
[17] White American patients were younger and had significantly more classic symptoms of the disease, including thrombosis, hemoglobinuria, and infection, whereas Asian patients were older with more marrow aplasia and a smaller PNH clone. Survival analysis showed a similar death rate in each group, although the causes of death were different, with more thrombotic deaths seen in the American patients. Japanese patients had a longer mean survival time (32.1 vs 19.4 y), but Kaplan-Meier survival curves were not significantly different.
[17]

Other geographic ethnic differences were observed in the thrombosis incidence in 64 patients with classic PNH.
[18] The investigators found that African Americans (n = 11) and Latin Americans (n = 8) had a higher risk or rate of thrombosis by Cox regression analysis and that this had an impact on length of survival compared with other patients (n = 45).

Sex- and Age-related Demographics

Men and women are affected equally with PNH, and no familial tendencies exist.

PNH may occur at any age, from children (10%) as young as 2 years to adults as old as 83 years, but median age at the time of diagnosis was 42 years (range, 16-75 y) in an English series of 80 consecutive patients.
[16] In childhood through adolescence, patients with PNH presented with more of the primary features of aplastic anemia than the healthy adult population. Other complications, such as infections and thrombosis, occurred with equal frequency in all age groups.

This series of containers holds urine of a patient with paroxysmal nocturnal hemoglobinuria, showing the episodic nature of the dark urine (hemoglobinuria) during intravascular hemolysis, usually occurring at night. Early morning urine is cola-colored. This may occur at different times of the day and vary from patient to patient. Permission to use this image has been granted by the American Society of Hematology Slide Bank, 3rd edition.